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IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 1 40v/1.2a led driver with internal switch general description the IS31LT3360 is a continuous mode inductive step-down converter, designed for driving a single led or multiple series connected leds efficiently from a voltage source higher than the led voltage. the chip operates from an input supply between 6v and 40v and provides an externally adjustable output current of up to1.2a. the IS31LT3360 includes an integrated output switch and a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current. output current can be adjusted linearly by applying an external control signal to the adj pin. the adj pin will accept either a dc voltage or a pwm waveform. this will provide either a continuous or a gated output current. applying a voltage less than 0.2v to the adj pin turns the output off and switches the chip into a low current standby state. the chip is assembled in sot89-5 package. features ? up to 1.2a output current ? high efficiency (up to 98% ) ? wide input voltage range: 6v to 40v ? internal 40v power switch ? simple low parts count ? typical 3% output current accuracy ? single pin on/off and brightness control using dc voltage or pwm ? up to 1mhz switching frequency ? inherent led open-circuit/sho rt-circuit protection ? thermal shutdown protection circuitry ? typical 1200 1 dimming ratio applications ? led mr16, mr11 spotlight ? led street light ? par light ? other led lighting application circuit IS31LT3360 october 2011 copyright ? ? ? 2011 ? integrated ? silicon ? solution, ? inc. ? all ? rights ? reserved. ? issi ? reserves ? the ? right ? to ? make ? changes ? to ? this ? specification ? and ? its ? products ? at ? any ? time ? without ? notice. ? issi ? assumes ? no ? liability ? arising ? out ? of ? the ? application ? or ? use ? of ? any ? information, ? products ? or ? services ? described ? herein. ? customers ? are ? advised ? to ? obtain ? the ? latest ? version ? of ? this ? device ? specification ? before ? relying ? on ? any ? published ? information ? and ? before ? placing ? orders ? for ? products. ? integrated ? silicon ? solution, ? inc. ? does ? not ? recommend ? the ? use ? of ? any ? of ? its ? products ? in ? life ? support ? applications ? where ? the ? failure ? or ? malfunction ? of ? the ? product ? can ? reasonably ? be ? expected ? to ? cause ? failure ? of ? the ? life ? support ? system ? or ? to ? significantly ? affect ? its ? safety ? or ? effectiveness. ? products ? are ? not ? authorized ? for ? use ? in ? such ? applications ? unless ? integrated ? silicon ? solution, ? inc. ? receives ? written ? assurance ? to ? its ? satisfaction, ? that: ? a.) ? the ? risk ? of ? injury ? or ? damage ? has ? been ? minimized; ? b.) ? the ? user ? assume ? all ? such ? risks; ? and ? c.) ? potential ? liability ? of ? integrated ? silicon ? solution, ? inc ? is ? adequately ? protected ? under ? the ? circumstances
IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 2 pin configurations package pin configurations sot89-5 pin description (note 1) pin name no. description lx 1 drain of power switch gnd 2 ground (0v) adj 3 multi-function on/off and brightness control pin: * leave floating for normal operation.(v adj = v ref = 1.2v giving nominal average output current i out nom =0.1/r s ) * drive to voltage below 0.2v to turn off output current * drive with dc voltage (0.3vIS31LT3360-sdls3 sot89-5, lead-free 2500
IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 3 absolute maximum ratings (note 2) symbol parameter rating v in input voltage -0.3v to +50v v isense i sense voltage v in +0.3v to v in -5v ,v in 5v v in +0.3v to -0.3v,v in <5v v lx lx output voltage -0.3v to +50v v adj adjust pin input voltage -0.3v to +6v i lx switch output current 1.5a p tot power dissipation 0.5w t op operating temperature -40 to 105c t st storage temperature -55 to 150c t j max junction temperature 150c r ja junction to ambient 100c/w esd rate(human body mode) 3kv electrical characteristics (v in =12v, t a =25c unless otherwise stated) (note 3) symbol parameter conditions min. typ. max. unit v in input voltage 6 40 v i inqoff quiescent supply current with output off adj pin grounded 90 120 160 a i inqon quiescent supply current with output switching adj pin floating 450 600 a v sense mean current sense threshold voltage 97 100 103 mv v sensehys sense threshold hysteresis 15 % i sense i sense pin input current vsense =vin -0.1 8 a v ref internal reference voltage measured on adj pin with pin floating 1.2 v v adj external control voltage range on adj pin for dc brightness control 0.3 1.2 v v adjoff dc voltage on adj pin to switch chip from active (on) state to quiescent (off) state v adj falling 0.15 0.2 0.25 v v adjon dc voltage on adj pin to switch chip from quiescent (off) state to active (on) state v adj rising 0.2 0.25 0.3 v r adj resistance between adj pin and v ref 500 k ? i lxmean continuous lx switch current 1.2 a
IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 4 electrical characteristics (v in =12v, t a =25c unless otherwise stated) (note3) (continued) symbol parameter conditions min. typ. max. unit i lx(leak) lx switch leakage current 1 a r lx lx switch ?on? resistance 0.27 0.4 ? t onmin minimum switch ?on? time lx switch ?on? 200 ns t offmin minimum switch ?off? time lx switch ?off? 200 ns ddim typical dimming ratio f=100hz vin=15v 1led l=27uh 1200:1 f lxmax recommended maximum operating frequency 1 mhz d lx recommended duty cycle range of output switch at f lxmax 0.3 0.7 0.9 t pd internal comparator propagation delay 50 ns t sd thermal shutdown temperature 150 c t sd-hys thermal shutdown hysteresis 20 c notes: 2. operation of the device at value exc eeding the absolute maximum ratings may caus e permanent damage to the device and reduce overall reliability. 3. production testing of the chip is performed at 25 c. functional operation of the chip and par ameters specified are guaranteed by design, characterization and process control at other temperature.
IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 5 typical operating conditions for typical application circuit and t a =25c unless otherwise stated. operating supply current vs. vin efficiency vs. no. of leds l=47uh, rs=0.10ohm efficiency vs. no. of leds l=47uh, rs=0.30ohm quiescent shutdown current vs. vin efficiency vs. no. of leds l=47uh, rs=0.15ohm output current variation with output voltage l=47uh, rs=0.15ohm 0 100 200 300 400 500 600 5 10152025303540 vin(v) icc(ua) rs=0.10 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 5 10152025303540 vin(v) effiency(%) 1led 3led 7led 10led rs=0.30 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 5 10152025303540 vin(v) effiency(%) 1led 3led 7led 10led 0 50 100 150 200 250 5 10152025303540 vin(v) icc(ua) rs=0.15 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 5 10152025303540 vin(v) effiency(%) 1led 3led 7led 10led rs=0.15 0.6 0.61 0.62 0.63 0.64 0.65 0.66 0.67 0.68 0.69 0.7 5 10152025303540 vin(v) iout(a) 1led 3led 7led 10led
IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 6 typical operating condition temperature vs vsense voltage l=47uh, rs=0.10ohm dimming rate current rise time=7.85us f=100hz square waveform in adj pin 97.8 98.0 98.2 98.4 98.6 98.8 99.0 99.2 99.4 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 ?() vsense(mv) 40vdc
IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 7 led open circuit protection vin=24vdc l=47uh, rs=0.30ohm led short circuit protection vin=24vdc l=47uh, rs=0.30ohm
IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 8 application information setting nominal average output current with external resistor r s the nominal average output current in the led(s) is determined by the value of the external current sense resistor (r s ) connected between v in and i sense and is given by: i out nom = 0.1/r s the table below gives values of nominal average output current for several preferred values of current setting resistor (r s ) in the typical application circuit shown on page 1: r s ( ? ) nominal average output current (ma) 0.082 1200 0.15 667 0.3 333 the above values assume that the adj pin is floating and at a nominal voltage of v ref =1.2v. note that r s =0.082 is the minimum allowed value of sense resistor under these conditions to maintain switch current below the specified maximum value. it is possible to use different values of r s if the adj pin is driven from an external voltage. rs need to be chosen 1% accuracy resistor with enough power tolerance and good temperature characteristic to ensure stable output current. output current adjustment by external dc control voltage the adj pin can be driven by an external dc voltage (v adj ), as shown, to adjust the output current to a value above or below the nominal average value defined by r s . the nominal average output current in this case is given by: i outdc = 0.083*v adj /r s [for 0.3v< v adj <1.2v] note that 100% brightness setting corresponds to v adj = v ref . when driving the adj pin above 1.2v, the current will be clamped to 100% brightness automatically. the input impedance of the adj pin is 500k ? 25%. output current adjustment by pwm control directly driving adj input a pulse width modulated (pwm) signal with duty cycle d pwm can be applied to the adj pin, as shown below, to adjust the output current to a value below the nominal average value set by resistor r s , the signal range is from 0v~5v. the logic ?high? is higher than 1.2v, the logic ?low? is lower than 0.2v. the pwm signal must have the driving ability to drive the internal 500k ? pull-up resistor. driving the adj input from a microcontroller another possibility is to drive the chip from the open drain output of a microcontroller. the diagram below shows one method of doing this: the diode and resistor suppress possible high amplitude negative spikes on the adj i nput resulting from the drain-source capacitance of the fet. negative spikes at the input to the chip should be avoided as they may cause errors in output current or erratic device operation. shutdown mode taking the adj pin to a voltage below 0.2v will turn off the output and supply current will fall to a low standby level of 120 a nominal. inherent open-circuit led protection if the connection to the led(s) is open-circuited, the coil is isolated from the lx pin of the chip, so the chip will not be damaged, unlike in many boost converters, where the back emf may damage the internal switch by forcing the drain above its breakdown voltage. capacitor selection a low esr capacitor should be used for input decoupling, as the esr of this capacitor appears in series with the supply source impedance and lowers overall efficiency. this capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. if the source is a dc supply, the capacitor is decided by ripple of the source, the value is given by:
IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 9 max on f u t i c ? ? * min i f is the value of output current max u ? is the ripple of power supply. ton is the ?on? time of mosfet. the value is higher than the minimum value. actually it is more than 47uf. if the source is an ac supply, typical output voltages ripple from a nominal 12v ac transformer can be 10%. if the input capacitor value is lower than 200 f, the ac input waveform is distorted, some times the lowest value will be lower than the forward voltage of led strings. this will lower the average current of the leds. so it is recommended to set the value of the capacitor bigger than 200uf. for maximum stability over temperature and voltage, capacitors with x7r, x5r, or better dielectric are recommended. capacitors with y5v dielectric are not suitable for decoupling in this application and should not be used. inductor selection recommended inductor values for the IS31LT3360 are in the range of 47 h to 220 h. higher values of inductance are recommended at higher supply voltages and low output current in order to minimize errors due to switching delays, which result in increased ripple and lower efficiency. higher values of inductance also result in a smaller change in output current over the supply voltage range. the inductor should be mounted as close to the chip as possible with low resistance connections to the lx and v in pins. the chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean output current. it is recommended to use inductor with saturation current bigger than 1.2a for 700ma output current and inductor with saturation current bigger than 500ma for 350ma output current, etc. the inductor value should be chosen to maintain operating duty cycle and switch 'on/off' times within the specified limits over the supply voltage and load current range. the following equations can be used as a guide. lx switch 'on' time ) ( lx s avg led in on r rl r i v v i l t ? ? ? ? ? ? note: t onmin >200ns lx switch 'off' time ) ( s avg d led off r rl i v v i l t ? ? ? ? ? note: t offmin >200ns where: l is the coil inductance (h) r l is the coil resistance ( ? ) i avg is the required led current (a) ? i is the coil peak-peak ripple current (a) {internally set to 0.3 i avg } v in is the supply voltage (v) v led is the total led forward voltage (v) r lx is the switch resistance ( ? ) v d is the diode forward voltage at the required load current (v) example: for v in =12v, l=47 h, r l =0.26 ? , v led =3.4v, i avg =333ma and v d =0.36v r lx =0.27 ? t on = (47e-6 0.105)/(12 - 3.4 - 0.274) = 0.59 s t off = (47e-6 0.105)/(3.4 + 0.36 + 0.188)= 1.25 s this gives an operating frequency of 543khz and a duty cycle of 0.32 optimum performance will be achieved by setting the duty cycle close to 0.5 at t he nominal supply voltage. this helps to equalize the undershoot and overshoot and improves temperature stability of the output current. diode selection for maximum efficiency and performance, the rectifier (d1) should be a fast low capacitance schottky diode with low reverse leakage at the maximum operating voltage and temperature. if alternative diodes are used, it is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. it is very important to consider the reverse leakage of the diode when operating at high temperature. excess leakage will increase the power dissipation in the device. the higher forward voltage and overshoot due to reverse recovery time in silicon di odes will increase the peak voltage on the lx output. if a silicon diode is used, care should be taken to ensure that the total voltage appearing on the lx pin including supply ripple, does not exceed the specified maximum value. reducing output ripple peak to peak ripple current in the led can be reduced, if required, by shunting a capacitor c3 across the led(s) as shown below:
IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 10 a value of 1 f will reduce nominal ripple current by a factor three (approx.). propor tionally lower ripple can be achieved with higher capacitor values. note that the capacitor will not affect opera ting frequency or efficiency, but it will increase start-up delay , by reducing the rate of rise of led voltage. operation at low supply voltage the internal regulator disables the drive to the switch until the supply has risen above the startup threshold set internally which makes power mosfet on-resistance small enough. above this thres hold, the chip will start to operate. however, with the supply voltage below the specified minimum value, the switch duty cycle will be high and the chip power dissipation will be at a maximum. care should be taken to avoid operating the chip under such conditions in the application, in order to minimize the risk of exceeding the maximum allowed die temperature. (see next section on thermal considerations ). note that when driving loads of two or more leds, the forward drop will normally be su fficient to prevent the chip from switching below appro ximately 6v. this will minimize the risk of damage to the chip. thermal considerations when operating the chip at high ambient temperatures, or when driving maximum load current, care must be taken to avoid exceeding the package power dissipation limits. it will also increase if the effi ciency of the circuit is low. this may result from the use of unsuitable coils, or excessive parasitic output capacitance on the switch output. layout considerations v in pin the gnd of power supply usually have some distance to the chip gnd pin, which cause parasitic resistance and inductance. it causes ground voltage bounce when the internal mosfet is switching. connect a 0.1uf capacitor c2 as close to device as possible to minimize the ground bounce. lx pin the lx pin of the chip is a fast switching node, so pcb traces should be kept as short as possible. to minimize ground 'bounce', the ground pin of the chip should be soldered directly to the ground plane. coil and decoupling capacitor c1 it is particularly important to mount the coil and the input decoupling capacitor close to the chip to minimize parasitic resistance and indu ctance, which will degrade efficiency. it is also important to take account of any trace resistance in series with current sense resistor r s . adj pin the adj pin is a high impedance input, so when left floating, pcb traces to this pin should be as short as possible to reduce noise pickup. adj pin can also be connected to a voltage between 1.2v~5v. in this case, the internal circuit will clam p the output current at the value which is set by adj=1.2v. high voltage traces avoid running any high voltage traces close to the adj pin, to reduce the risk of leakage due to board contamination. any such leakage may affect the adj pin voltage and cause unexpected output current. the IS31LT3360 has external protection circuitry to prevent excessive output current if adj voltage raise above 1.2v. a ground ring placed around the adj pin will minimize changes in output current under these conditions.
IS31LT3360 integrated silicon solution, inc. ? www.issi.com rev. a, 10/19/2011 11 package information sot89-5

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